Frameworks whereby the junction members have several connecting pieces for the uniform bar elements are known, e.g. from German Printed Specification No. 28 19 632, said connecting pieces forming three-dimensional or plane angles. In the open ends of the connecting pieces threaded holes are provided, centrally punched plates are mounted at the end of the bar element, and a cap screw is inserted in each punched plate with its head positioned inside the bar element. The screw is screwable into the threaded holes on the side of the connecting piece. Finally the bar elements, the plates and the connecting pieces have a smooth exterior profiling in order to yield a framework free of recesses and projections.
In the case of the German reference a cap screw with a polygonal shank portion is provided, over which a coupling piece in the shape of a plate, cooperating with the polygonal shank portion is slipped. This coupling piece can be acted upon from the outside by means of a wrench or the like, permitting the tightening of the screw.
However, in this embodiment one cannot apply sufficient tightening force to the screw, since it is not possible to apply a large enough lever arm for the actuation of the coupling part.